Recording media, data storage devices, and methods for determining a position error signal in a recording medium

ABSTRACT

According to various embodiments, a recording medium may be provided. The recording medium may include a dedicated servo layer configured to provide servo information. The dedicated servo layer may include a plurality of tracks. A first track may include a first servo signal. The first servo signal may include first servo bursts of a pre-determined frequency. A second track adjacent to the first track may include a second servo signal. The second servo signal may include second servo bursts of the pre-determined frequency.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of the U.S. provisionalpatent application No. 61/673,759 filed on 20 Jul. 2012 and of the U.S.provisional patent application No. 61/673,764 filed on 20 Jul. 2012, theentire contents of which are incorporated herein by reference for allpurposes.

TECHNICAL FIELD

Embodiments relate generally to recording media, data storage devices,and methods for determining a position error signal in a recordingmedium.

BACKGROUND

Electronic devices, including mobile computing and/or communicationdevices, are becoming smaller thereby driving the weight and size ofdata storage devices down, while requiring large storage capacity in theterabyte range and low power consumption. An increasing storage capacitywould require the need for increased precision in tracking the movementof the read/write head.

Data storage devices, for example hard disk drives (HDDs), employ servosystems for tracking and controlling the movement of the read/writehead. Conventional servo systems may employ embedded servo where theservo information runs radially across the tracks from the innerdiameter (ID) to the outer diameter (OD) of the disk in a series of“servo wedges” with data. Therefore, the servo information is onlydetected when the read/write head moves over these servo wedges. Inbetween the servo wedges, no servo information is received by the head.

Data storage devices also employ dedicated servo, e.g. as shown in FIG.1, where the servo information is provided on a servo layer 104 distinctfrom the data recording layer 102 (in other words: data layer 102).

SUMMARY

According to various embodiments, a recording medium may be provided.The recording medium may include a dedicated servo layer configured toprovide servo information. The dedicated servo layer may include aplurality of tracks. A first track may include a first servo signal. Thefirst servo signal may include first servo bursts of a pre-determinedfrequency. A second track adjacent to the first track may include asecond servo signal. The second servo signal may include second servobursts of the pre-determined frequency.

According to various embodiments, a data storage device may be provided.The data storage device may include a recording medium according tovarious embodiments.

According to various embodiments, a method for determining a positionerror signal in a recording medium may be provided. The recording mediummay include: a dedicated servo layer configured to provide servoinformation. The dedicated servo layer may include a plurality oftracks. A first track may include a first servo signal. The first servosignal may include first servo bursts of a pre-determined frequency. Asecond track adjacent to the first track may include a second servosignal. The second servo signal may include second servo bursts of thepre-determined frequency. The method may include: reading a signal fromthe recording medium; and determining a position error signal based onthe read signal.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. The drawings are not necessarilyto scale, emphasis instead generally being placed upon illustrating theprinciples of the invention. In the following description, variousembodiments are described with reference to the following drawings, inwhich:

FIG. 1 shows a dedicated servo system;

FIG. 2 shows a diagram illustrating a DC (direct current) shift of a PES(position error signal) at a dual-frequency servo system;

FIG. 3A shows a recording medium according to various embodiments:

FIG. 3B shows a data storage device according to various embodiments;

FIG. 3C shows a flow diagram illustrating a method for determining aposition error signal in a recording medium according to variousembodiments;

FIG. 4A and FIG. 4B show a servo-pattern layout for dedicated servoaccording to various embodiments;

FIG. 5 shows an orthogonal servo pattern according to variousembodiments;

FIG. 6A shows a configuration of a synchronized average detectoraccording to various embodiments;

FIG. 6B shows an illustration of plots of data processed in thesynchronized average detector;

FIG. 7A shows a diagram illustrating the NRRO (non-repeatable runout) ofa dual frequency servo;

FIG. 7B shows a diagram illustrating the NRRO of the orthogonal servopattern according to various embodiments;

FIG. 8 shows an illustration of a differential servo bursts layout fordedicated servo according to various embodiments;

FIG. 9 shows a differential servo pattern according to variousembodiments;

FIG. 10 shows an illustration of the differential servo bursts accordingto various embodiments;

FIG. 11A and FIG. 11B show simulation results according to variousembodiments;

FIG. 12 shows a screen shot of a simulation;

FIG. 13A shows a diagram illustrating a relationship between a sectornumber and a peak to peak amplitude of an averaged cycle for a frequencyof 50 MHz and various heater powers;

FIG. 13B shows a diagram illustrating a relationship between a sectornumber and a peak to peak amplitude of an averaged cycle for a frequencyof 80 MHz and various heater powers;

FIG. 14A shows a diagram illustrating a relationship between a sectornumber and a peak to peak amplitude of an averaged servo cycle;

FIG. 145 shows a diagram illustrating a cross-correlation of a servo rawsignal with a 50 MHz sinusoid;

FIG. 15A shows a diagram illustrating raw data and 100 cycles AGCnormalized data;

FIG. 15B shows a diagram illustrating raw data and 200 cycles AGCnormalized data;

FIG. 15C shows a diagram illustrating raw data and 300 cycles AGCnormalized data; and

FIG. 15D shows a diagram illustrating raw data and 500 cycles AGCnormalized data.

DESCRIPTION

Embodiments described below in context of the devices are analogouslyvalid for the respective methods, and vice versa. Furthermore, it willbe understood that the embodiments described below may be combined, forexample, a part of one embodiment may be combined with a part of anotherembodiment.

In this context, the data storage device as described in thisdescription may include a memory which is for example used in theprocessing carried out in the data storage device. A memory used in theembodiments may be a volatile memory, for example a DRAM (Dynamic RandomAccess Memory) or a non-volatile memory, for example a PROM(Programmable Read Only Memory), an EPROM (Erasable PROM), EEPROM(Electrically Erasable PROM), or a flash memory, e.g., a floating gatememory, a charge trapping memory, an MRAM (Magnetoresistive RandomAccess Memory) or a PCRAM (Phase Change Random Access Memory).

In an embodiment, a “circuit” may be understood as any kind of a logicimplementing entity, which may be special purpose circuitry or aprocessor executing software stored in a memory, firmware, or anycombination thereof. Thus, in an embodiment, a “circuit” may be ahardwired logic circuit or a programmable logic circuit such as aprogrammable processor, e.g. a microprocessor (e.g. a ComplexInstruction Set Computer (CISC) processor or a Reduced Instruction SetComputer (RISC) processor). A “circuit” may also be a processorexecuting software, e.g. any kind of computer program, e.g. a computerprogram using a virtual machine code such as e.g. Java. Any other kindof implementation of the respective functions which will be described inmore detail below may also be understood as a “circuit” in accordancewith an alternative embodiment.

Electronic devices, including mobile computing and/or communicationdevices, are becoming smaller thereby driving the weight and size ofdata storage devices down, while requiring large storage capacity in theterabyte range and low power consumption. An increasing storage capacitywould require the need for increased precision in tracking the movementof the read/write head.

In hard disk drives, a servo system may be very important. It maycontrol the moving of the read/write head on the disk. It may use servopatterns to position the read/write head on the center of the track toread and write user data. In commonly used hard disk drives, servoinformation may be recorded in servo wedges. The performance of trackingsystems may be limited by the servo sampling rate. The higher the servosampling rate the better the tracking performance. The samplingfrequency of the servo system may be limited by the number of servowedges in one revolution and the rotating speed of the disk.

Data storage devices, for example hard disk drives (HDDs), employ servosystems for tracking and controlling the movement of the read/writehead. Conventional servo systems may employ embedded servo where theservo information runs radially across the tracks from the innerdiameter (ID) to the outer diameter (OD) of the disk in a series of“servo wedges” interspersed with data. Therefore, the servo informationis only detected when the read/write head moves over these servo wedges.In between the servo wedges, no servo information is received by thehead.

To improve the tracking accuracy, the number of servo wedges may bedesired to be increased. However, increase the number of servo wedgesmay reduce the number of data sector for user data, and thus may reducethe capacity of the hard disk drive. A dedicated servo system may combatthis problem.

HCl. 1 shows a dedicated servo system 100. For example in a data storagedevice with the dedicated servo system 100, dedicated servo may beemployed, where the servo information is provided on a servo layer 104distinct from the data recording layer 102 (in other words: data layer102).

In a dedicated servo implementation, one disk surface (servo layer) isdedicated to store the position data referred to as servo data or servosignal. The servo layer may be a buried layer arranged beneath the datarecording layer and may have a perpendicular or a longitudinalmagnetization orientation for providing magnetic information fordetermining the location of the head in relation to the storage medium.The servo information is provided on the servo layer distinct from thedata recording layer so as to allow continuously available servoreadback to enable continual position feedback thereby providingcontinual position detection without utilizing any of the recordinglayer for position detection. This may provide higher positioningaccuracy through continual location determination, while also removingthe servo sectors/tracks from the recording layer, thereby increasingsurface utilization of the storage space in the recording layer andfurther increasing the data recording track density by increasing thetrack positioning accuracy. Further, the servo layer and data layer maybe put on the same side and they may be read and/or processed together;this may also be referred to as dedicated servo.

The dedicated servo layer may have a continuous track structure having aplurality of servo tracks in a concentric arrangement.

In the dedicated servo system 100, a magnetic disk with two magneticlayers 102 and 104 may be used. The top layer 102 (the data layer) mayhave less spacing loss, and may be used to record user data. The bottomlayer 104 (the servo layer) may be used to record servo pattern becauseservo layer may support low linear density.

In the dedicated servo system, data and servo may be written atdifferent layers. The top layer 102 may have a high linear density andmay be used to write user data. The bottom layer 104 may have a lowlinear density and may be used to write servo pattern. The dedicatedservo system may provide continuous PES (position error signal)information and thus may improve the servo performance.

In dedicated servo systems, servo and data are written at differentlayers. In one example, dual-frequency servo bursts may be written onthe servo layer to provide continuous positioning error signal (PES). Asingle tone servo signal f₁ may be written at one track and anothersingle tone signal f₂ may be written at adjacent track. The signalamplitude of f₁ and f₂ may be estimated as A and B, respectively. Theratio of (A−B)/(A+B) may be used as the positioning error signal (PES)for the servo. The dedicated servo system may provide continuous PESsignal and thus can improve the servo performance.

Dual-frequency servo systems may provide continuous PES and thus mayimprove servo performance in HDD. Dual-frequency servo systems may usetwo different single tones f₁ and f₂ (in other words: on each twoneighboring tracks of the servo layer, one track has a signal of a firstfrequency, and another track has a signal of a second frequency, andtracks with the first frequency and the second frequency are providedalternatingly) to generate the continuous positioning error signal(PES). However, the performance of the dual-frequency servo may bedegraded due to the coupling of data signal to the servo. Simulation mayshow that the performance of the RRO (repeatable runout) may be affectedby a 90 Hz harmonic (for RPM (rotations per minute) 7200 drive, theharmonic may be 120 Hz). This harmonic may be caused by the repeat ofthe coupled data.

In the dedicated servo system, interference from data layer may be a bigproblem to PES demodulation. It may affect the quality of PES signal byinducing significant repeatable interference components in the PESdemodulation, which may significantly affect the servo trackingperformance.

The performance of the dual-frequency system may be affected by theinterference from data layer. In order to read back data correctly, thesignal at the data layer may be stronger than servo signal. This maycause a serious interference to servo and thus reduce the servoperformance. In 5400 RPM drive, a 90 Hz harmonic (for 7200 RPM drive,the harmonic is 120 Hz) may be observed in Repeatable Run-Out (RRO).This harmonic may be caused by the repeat of the coupled data. Theperformance of RRO may be degraded due to this harmonic.

A notch filter may be used to reduce the interference. However, the (BER(bit error rate)) performance of the data channel may still be affectedor degraded by this notch filter.

Another drawback of the dual-frequency servo system (for exampledual-frequency servo bursts for dedicated servo) may be that the spacingloss at different frequency may be different. The spacing losscompensation may be needed to be done in order to cancel the DC (directcurrent) shift (at the PES). This spacing loss difference may cause a DCshift at PES, as shown in FIG. 2. The spacing loss at differentfrequency may be different, and thus the spacing loss compensation mustbe done for the dual-frequency servo burst.

FIG. 2 shows a diagram 200 illustrating the DC shift of (or at) the PESat a dual-frequency servo system. This DC shift may be caused (orinduced) by the different spacing loss at different frequency. Curve 202shows the real RRO in time domain, while curve 204 shows the ideal RROin time domain.

The different spacing loss may induce a DC shift in PES and maymisinterpret the FH (fly height) variations due to disk surface andoutplace motions into estimated PES as an input disturbance.

FIG. 3A shows a recording medium 300 according to various embodiments.The recording medium 300 may include a dedicated servo layer 302configured to provide servo information. The dedicated servo layer 300may include a plurality of tracks (not shown in FIG. 3A). A first trackmay include a first servo signal. The first servo signal may includefirst servo bursts of a pre-determined frequency. A second trackadjacent to the first track may include a second servo signal. Thesecond servo signal may include second servo bursts of thepre-determined frequency.

In other words, in a dedicated servo layer, adjacent servo tracks mayhave servo bursts with signals of the same frequency.

According to various embodiments, the first servo bursts may beorthogonal to the second servo bursts.

According to various embodiments, the first servo bursts may include asine signal. According to various embodiments, the second servo burstsmay include a sine signal, which is phase shifted compared to the firstservo bursts, for example shifted by 90 degrees.

According to various embodiments, the first servo bursts may include asine signal. According to various embodiments, the second servo burstsmay include a cosine signal.

According to various embodiments, the first servo bursts may be negated(in other words: inverted; in other words: differential; in other words:multiplied by minus one; in other words: multiplied by −1) to the secondservo bursts.

According to various embodiments, the first servo bursts may include asine signal. According to various embodiments, the second servo burstsmay include a sine signal, which is phase shifted compared to the firstservo bursts, for example shifted by 180 degrees.

According to various embodiments, the first servo bursts may include asine signal. According to various embodiments, the second servo burstsmay include a negated sine signal (in other words: an inverted sinesignal; in other words: a differential sine signal; in other words: asine signal multiplied by minus one; in other words: a-sine signal).

According to various embodiments, the first servo signal may include aplurality of alternately arranged preambles and servo bursts. Accordingto various embodiments, the second servo signal may include a pluralityof alternately arranged preambles and servo bursts.

According to various embodiments, the first servo signal and the secondservo signal may include information for providing positioninginformation.

FIG. 3B shows a data storage device 304 according to variousembodiments. The data storage device 304 may include a recording mediumaccording to various embodiments, for example the recording medium 300of FIG. 3A described above. The data storage device 304 may furtherinclude a reader head 306, like will be described in more detail below.The data storage device 304 may further include in position error signaldetermination circuit 308, like will be described in more detail below.The recording medium 300, the reader head 306, and the position errorsignal determination circuit 308 may be coupled with each other, likeindicated by lines 310, for example electrically coupled, for exampleusing a line or a cable, and/or mechanically coupled.

According to various embodiments, the recording medium 300 may furtherinclude a data layer configured to record data therein.

According to various embodiments, the reader head 306 may be configuredto read a signal from the recording medium 300.

According to various embodiments, the position error signaldetermination circuit 308 may be configured to determine a positionerror signal based on the read signal.

According to various embodiments, the position error signaldetermination circuit 308 may further be configured to determine theposition error signal based on an average of the read signal over aplurality of servo cycles.

According to various embodiments, the position error signaldetermination circuit 308 may further be configured to determine theposition error signal based on multiplying the read signal with awaveform of the pre-determined frequency.

FIG. 3C shows a flow diagram 312 illustrating a method for determining aposition error signal in a recording medium according to variousembodiments. The recording medium may include a dedicated servo layerconfigured to provide servo information. The dedicated servo layer mayinclude a plurality of tracks. A first track may include a first servosignal. The first servo signal may include first servo bursts of apre-determined frequency. A second track adjacent to the first track mayinclude a second servo signal. The second servo signal may includesecond servo bursts of the pre-determined frequency. In 314, a signalmay be read from the recording medium. In 316, a position error signalmay be determined based on the read signal.

According to various embodiments, the position error signal may bedetermined based on an average of the read signal over a plurality ofservo cycles.

According to various embodiments, the position error signal may bedetermined based on multiplying the read signal with a waveform of thepre-determined frequency.

According to various embodiments, the first servo bursts may beorthogonal to the second servo bursts.

According to various embodiments, the first servo bursts may be negatedto the second servo bursts (in other words: may be inverted; in otherwords: may be differential; in other words: may be multiplied by minusone; in other words: may be multiplied by −1).

According to various embodiments, servo control systems in disk drivesmay be provided.

According to various embodiments, orthogonal servo bursts for digitalPES demodulation for dedicated servo may be provided.

According to various embodiments, a synchronized average detector and anorthogonal servo pattern may overcome the above stated problems ofcommonly used devices and methods.

FIG. 4A and FIG. 4B show a servo-pattern layout for dedicated servoaccording to various embodiments. In illustration 400 of FIG. 4A, a datalayer 402 and a servo layer 404 are shown. An auto gain control (AGC)406, a sector address mark (SAM) 408, a gray code 410, data 412 andorthogonal servo bursts 414 are shown. In FIG. 4B, an illustration 416illustrates the distribution of AGC 406, SAM 408, gray code 410, andservo bursts 414 in down-track direction 418 and off-track direction 420(for example from outer diameter OD to inner diameter ID). AGC 406, SAM408, and gray code 410 may provide coarse position for seeking, whilethe servo bursts 414 may provide fine position for track following.

According to various embodiments, a single tone f₁ may be written at onetrack and an orthogonal wave form of f₁ may be written at another (forexample neighboring) track, as shown in FIG. 5.

FIG. 5 shows an orthogonal servo pattern 500 (in other words: anorthogonal layout of a servo pattern) according to various embodiments.A down track direction 502 and an off track direction 504 are shown. Apreamble portion 518 may be provided on each track (for example anidentical preamble on each track). Servo bursts 520 may be provided in asubsequent portion of each track. A single tone (for example of afrequency f₁) may be written at one track, and the orthogonal waveformmay be written at another track. The PES may then be estimated by simpleAM (amplitude modulation) demodulation.

For example, a first signal A may be written on a k-th track 506 (withan integer number k). The signal A may for example be a sine signal, forexample according to a sin(ωt), like indicated by formula 514 in FIG. 5,with amplitude a and angular velocity ω. For example, a second signal Bmay be written on a k+1-th track 508. The signal B may for example be acosine signal, for example according to b cos(ωt), like indicated byformula 516 in FIG. 5, with amplitude b and angular velocity ω. Forexample, the first signal A may then be provided again at the subsequentk+2-th track 510. For example, the second signal B may then be providedagain at the subsequent k+3-th track 512. In other words: the orthogonalsignals A and B may be provided alternatingly on neighboring tracks.

The orthogonal servo pattern according to various embodiments mayeliminate the need to do spacing loss compensation. Performance analysisand simulation results show that the orthogonal servo pattern accordingto various embodiments may outperform dual-frequency system in NRRO(non-repeatable runout) performance.

According to various embodiments, a synchronized average detector may beprovided. The synchronized average detector according to variousembodiments may reduce the interference from data layer

FIG. 6A shows a configuration of a synchronized average detector 600according to various embodiments. The synchronized average detector 600may for example be used for orthogonal servo patterns according tovarious embodiments or for differential servo patterns according tovarious embodiments. A flash ADC (analog digital converter) 602 mayprovide a read back signal. The read back signal may be synchronized andaveraged in a synchronization and averaging circuit 604 over a pluralityof (for example hundreds of) servo cycles to reduce the interferencefrom data layer as well as the noise. The result may then be multipliedin multiplier 606 with a reference signal of f₁. The amplitude of theservo signal may be estimated by doing integration over one servo cyclein integrator 608. The synchronized average detector 600 may make fulluse of the continuous servo pattern. For example, for the number of theaveraged servo cycle being 200, the SNR gain over the coupled data maybe 23 dB.

FIG. 6B shows an illustration 610 of plots of data processed in thesynchronized average detector 600. A read back signal 612 is shown. Anaveraged read back signal for one servo cycle (for example from hundredsof servo cycles) 614 is shown. A reference signal 616 with the samefrequency as the servo signal is shown. Amplitude of the (averaged)servo signal 614 may be estimated by multiplying the averaged servosignal 614 and the reference signal 616, for example in the sense of apoint-wise multiplication and then integration of the product, likeindicated by integration sign 618.

An orthogonal servo system according to various embodiments may bedescribed as follows: write one single tone f₁ at one track, write anorthogonal waveform of f₁ at another track. Assume the amplitude of f₁and its quadric signal are A and B respectively. A and B may beestimated using the mentioned synchronized average detector according tovarious embodiments. PES can be expressed as (A−B)/(A+B). By using thesynchronized average detector, the interference from the coupled datamay be reduced. Assuming that the number of average is 200, the signalto interference ration (SIR) gain is around 23 dB.

The performance of the orthogonal servo pattern according to variousembodiments may be analyzed as follows: The read back signal y of oneservo cycle may be expressed in vector format as

y=aw ₁ +bw ₂ +n.

wherein a is an amplitude of the first signal, w₁ is the normalizedfirst signal, b is an amplitude of the second signal, w₂ is thenormalized second signal, and n is noise. w₁ and w₂ may be orthogonalaccording to various embodiments.

The amplitude can be estimated by multiplying the read back signal withthe normalized first signal and with the normalized second signal,respectively. Multiplication of the read back signal with the normalizedfirst signal may yield the an estimate a of the amplitude a of the firstsignal as:

â=w ₁ ^(H) y=a+bw ₁ ^(H) w ₂ +w ₁ ^(H) n

and multiplication of the read back signal with the normalized secondsignal may yield an estimate {circumflex over (b)} of the amplitude b ofthe second signal as

{circumflex over (b)}=w ₂ ^(H) y=b+aw ₂ ^(H) w ₁ +w ₂ ^(H) n.

Then, PES may then be expressed as a normalized difference between theestimate a of the amplitude a of the first signal and the estimate{circumflex over (b)} of the amplitude b of the second signal. Forexample, the PES may be determined as:

${pes} = \frac{\hat{a} - \hat{b}}{\hat{a} + \hat{b}}$

It is obvious that if w₁ and w₂ are orthogonal (w₁ ^(H)w₂=0), the aboveexpression may be optimized (since the interference items bW₁ ^(H)W₂ andaW₂ ^(H)W₁ are all zeros) and may have the best performance (in otherwords: the estimate may be a good or the best possible estimate). Thus,the orthogonal servo pattern according to various embodiments mayoutperform dual-frequency servo system and the dual-frequency servoburst in reducing the effect of interference.

As described above, the digital PES demodulation may determine anestimate a of the amplitude a of the first signal and an estimate{circumflex over (b)} of the amplitude b of the second signal. Forexample, if the first signal is a sine signal, and the second signal isa cosine signal, the readback signal may be

signal_(readback) =a sin(ωt)+b cos(ωt)+n+data

if for example additionally the data is taken into account. Then, theestimate a of the amplitude a of the first signal may be derived as:

$\hat{a} = {\frac{\int_{0}^{T}{{signal}_{readback}{\sin \left( {\omega \; t} \right)}}}{T}*2}$

wherein T is the length of a servo cycle, and the estimate {circumflexover (b)} of the amplitude b of the second signal may be derived as:

$\hat{b} = {\frac{\int_{0}^{T}{{signal}_{readback}{\cos \left( {\omega \; t} \right)}}}{T}*2}$

By synchronized averaging, both the noise n and the data may be averagedout over hundreds of servo cycles for the determination of the estimatesof the amplitudes. However, the HER (bit error rate) performance of datachannel may be not degraded, as the data signal may be recovered asfollows:

data_(decoupled)=signal_(readback) −â sin(ωt)−{circumflex over (b)}cos(ωt)

The orthogonal servo pattern according to various embodiments mayeliminate the need to do spacing loss compensation for dual-frequencyservo system, since orthogonal servo pattern according to variousembodiments includes a single tone servo bursts which have the exactlysame spacing loss factor.

In the following, an analysis of the NRRO will be described.

FIG. 7A shows a diagram 700 illustrating the NRRO of a dual frequencyservo (for example with a first frequency of 50 MHz and a secondfrequency of 60 MHz).

FIG. 7B shows a diagram 702 illustrating the NRRO of the orthogonalservo pattern according to various embodiments. It can be observed thatthe NRRO performance of orthogonal servo is much better than theconventional wedge based servo system. And it can also outperform thedual-frequency servo system. The results of the orthogonal servo patternaccording to various embodiments may be better than conventional wedgebased servo system (the NRRO for the wedge based convention servo systemmay be around 10⁻³, while the orthogonal servo pattern according tovarious embodiments plus synchronized average detector may be only0.5*10⁻⁴). It is to be noted that since only one single tone is used,there is no need to do spacing loss compensation any more.

The NRRO performance of the synchronized average detector and theorthogonal servo pattern according to various embodiments may be0.5*10⁻⁴, like stated above, and thus may be better than synchronizedaverage detector and a dual-frequency servo pattern. Furthermore, itsperformance may be better than normal wedge based servo system. A reasonmay be that a synchronized average detector may make full use of thecontinuous servo pattern.

According to various embodiments, a synchronized average detector may beprovided to effectively reduce demodulation noise.

According to various embodiments, the synchronized average method mayalso effectively reduce the PES demodulation noise caused by thehead/media noise.

According to various embodiments, orthogonal servo bursts according tovarious embodiments may solve the problem of different spacing loss. TheNRRO performance may also be improved due to the orthogonal property ofthe servo bursts.

The orthogonal servo burst according to various embodiments may providea method to decouple servo signal from data signal.

According to various embodiments, for buried servo (wherein for examplethe servo layer is provided underneath the data layer on the same disk),the phase alignment may be done at the AGC part, and the orthogonalservo pattern may be implemented.

According to various embodiments, the reader may capture the signal fromtwo adjacent servo tracks, thus the PES detection may become thedetection of the amplitude of a single tone signal.

According to various embodiments, a synchronized average detector may beprovided. This detector may reduce the effect of interference and medianoise.

According to various embodiments, for orthogonal servo burst, there maybe no need to do spacing loss compensation, since they are using thesame single tone signal.

Performance analysis and simulation results demonstrate that theorthogonal servo burst according to various embodiments may outperformdual-frequency servo burst.

According to various embodiments, differential servo bursts for digitalPES demodulation for dedicated servo may be provided.

FIG. 8 shows an illustration 800 of a differential servo bursts layoutfor dedicated servo according to various embodiments. In illustration800, a data layer 802 and a servo layer 804 are shown. An auto gaincontrol (AGC) 806, a sector address mark (SAM) 808, a gray code 810,data 812 and differential servo bursts 814 are shown.

According to various embodiments, a single tone f₁ (in other words: asignal of one frequency f₁) may be written at one track, and adifferential wave form, for example a differential waveform of f₁ (inother words: a waveform of the same frequency f₁), may be written atanother track (for example at an adjacent track), as shown in FIG. 9.

FIG. 9 shows a differential servo pattern 900 (in other words: adifferential layout of a servo pattern; in other words: a configurationof the differential servo system) according to various embodiments. Adown track direction 902 and an off track direction 904 are shown. Apreamble portion 918 may be provided on each track (for example anidentical preamble on each track). Servo bursts 920 may be provided in asubsequent portion of each track. A single tone (for example of afrequency f₁) may be written at one track, and the differential waveformmay be written at another track. The PES may then be estimated by simpleAM (amplitude modulation) demodulation.

For example, a first signal A may be written on a k-th track 906 (withan integer number k). The signal A may for example be a sine signal, forexample according to a sin(ωt), like indicated by formula 914 in FIG. 9,with amplitude a and angular velocity ω. For example, a second signal Bmay be written on a k+1-th track 908. The signal B may for example be anegative sine signal, for example according to b −sin(ωt), likeindicated by formula 916 in FIG. 9, with amplitude b and angularvelocity ω. For example, the first signal A may then be provided againat the subsequent K+2-th track 910. For example, the second signal B maythen be provided again at the subsequent k+3-th track 912. In otherwords: the differential signals A and B may be provided alternatingly onneighboring tracks.

FIG. 10 shows an illustration 1000 of the differential servo burstsaccording to various embodiments. A k-th track 1002 and a k+1-th track1004 are shown. In a first portion 1006 of the k-th track 1002, a firstwaveform may be provided (for example indicated by N, for exampleindicating magnetic North pole), and in a second portion 1008 of thek-th track 1002 a second waveform may be provided (which for example isthe negative of the first wave form, for example indicated by S, forexample indicated magnetic South pole). In the adjacent k+1-th track1004, in the first portion 1010, the second waveform may be provided,and in the second portion 1012, the first waveform may be provided.

In other words, according to various embodiments, in the differentialservos bursts, signals of the same frequency for A and B of signals foradjacent servo tracks may be provided, but with a different phase (forexample with a 180 degree phase shift).

The differential servo pattern according to various embodiments mayovercome the problems of a two frequency servo pattern.

The differential servo pattern according to various embodiments mayreduce interference from coupled data and thus may reduce the 90 Hzharmonic in RRO. Performance analysis shows that, compared with thedual-frequency system, the differential servo pattern according tovarious embodiments may have 3 dB SNR gain.

The differential servo pattern according to various embodiments mayreduce the interference from servo layer to data channel. When on track,the interference from the servo layer may be completely cancelled. Thus,using the differential servo pattern according to various embodimentsmay improve the performance of the data channel.

Compare with the dual-frequency system, the differential servo patternaccording to various embodiments may also eliminate the need to dospacing loss compensation and thus may reduce the complexity of theservo system.

The differential servo system according to various embodiments may bedescribed as follows: one single tone f₁ may be written at one track,the differential waveform of f₁ may be written at another track. Assumethe amplitude of f₁ may be expressed as A. Assume the amplitude of thedifferential signal of f₁ may be expressed as B. The amplitude of theservo signal may be expressed as (A−B). Assume (A+B) may be normalizedas 1. According to various embodiments, PES demodulation may then becomethe detection of the amplitude A−B of the servo signal, in other words,the detection of the amplitude of the signal to which both adjacentservo tracks contribute.

The performance of the differential servo pattern according to variousembodiments (for example with respect to demodulation noise) may beanalyzed as follows: The read back signal y (which may be a N times 1vector, wherein N may be the number of samples used for describing oneservo cycle) of one servo cycle (for example read back from ADC602 ofFIG. 6A) may be expressed in vector format as

y=(A−B)w+n

wherein A may be the amplitude of the servo signal, B may be theamplitude of the differential servo signal, w may be a normalized servosignal (for example the sine function in the embodiment describedabove), and n may be noise.

By multiplying y with the reference signal w, the following result r maybe obtained:

r=w ^(H) y=(a−b)w ^(H) w+w ^(H) n=(a−b)+w ^(H) n

Thus, the noise power may be calculated as:

w ^(H) n×(w ^(H) n)^(H)=δ².

where δ² is the noise power.

For orthogonal or dual-frequency servo system, the read back signal maybe expressed as

y ₂ =aw ₁ +bw ₂ +n

As described above, the amplitudes of the respective contribution of thetwo servo tracks may be estimated as

â=w ₁ ^(H) y ₂ =a+w ₁ ^(H) n

and

{circumflex over (b)}=w ₂ ^(H) y ₂ =b+w ₂ ^(H) n

Then, the PBS may be expressed as

â−{circumflex over (b)}=(a−b)+(w ₁ ^(H) −w ₂ ^(H))n

It is obvious that the noise power for the dual frequency servo or forthe orthogonal servo may be expressed as:

(w ₁ ^(H) −w ₂ ^(H))n×((w ₁ ^(H) −w ₂ ^(H))n)^(H)=2δ²

Thus, the differential servo pattern according to various embodimentsmay have 3 dB SNR (signal to noise ratio) gain over orthogonal ordual-frequency servo system.

According to various embodiments, for example when w is a sine function,the signal processing for digital PES demodulation may be as follows.The read back signal may be

signal_(readback)=(a−b)sin(ωt)+n+data

including the data signal.

By multiplying with the sine function and integrating over one servopattern, an estimate for the amplitude of the feedback signal may beobtained as follows:

$= {\frac{\int_{0}^{T}{{signal}_{readback}{\sin \left( {\omega \; t} \right)}}}{T}*2}$

The PES may then for example be determined as

${pes} = \frac{\hat{a} - \hat{b}}{\hat{a} + \hat{b}}$

Assume that a+b is normalized as 1 and the fly height is constant, thedigital PES demodulation may be simplified to estimate the amplitude ofthe sinusoidal at one frequency.

It is to be noted that, when the head is on track (for example about inthe middle between two servo tracks, for example a=b), the servo signalcomponent may be minimized (or about zero) in the read back signal whichmay be fed into data channel for data process directly.

FIG. 11A and FIG. 11B show simulation results according to variousembodiments. In the comparison of the RRO in PES noise, FIG. 11A shows adiagram 1100 illustrating simulation results for the RRO of theorthogonal servo according to various embodiments, and FIG. 11B shows adiagram 1102 illustrating simulation results for the RRO of thedifferential servo according to various embodiments. It may be seen thatthe RRO performance of the differential servo system according tovarious embodiments is better. Parameter K may stand for the ratiobetween data and servo, K=5 may mean that data signal is 5 times asstrong as the servo signal. The results demonstrate that with the helpof the synchronized average detector and differential servo pattern, theservo performance is acceptable even if the data signal is much strongerthan the servo signal.

As can be seen from the RRO performance comparison between orthogonalservo in FIG. 11A and differential servo system in FIG. 11B, thedifferential servo according, to various embodiments may have betterperformance than orthogonal servo system according to variousembodiments. The RRO performance of the differential servo patternaccording to various embodiments may outperform the orthogonal servopattern system according to various embodiments and the dual-frequencyservo system. The reason may be that the orthogonal servo pattern has 3dB SNR gain over the orthogonal servo pattern system according tovarious embodiments and the dual-frequency servo system.

It is to be noted, that when on track (A=B), the interference from servolayer to data channel may be 0. Thus the performance of the data channelmay be improved.

The differential servo pattern according to various embodiments mayeliminate the needs to do spacing loss compensation. In dual-frequencyservo system, the spacing loss at different frequency is different. Thismay cause a DC shift at PES as described above. To solve this problem,spacing loss compensation is required. By using the differential servosystem according to various embodiments, there may be no need to performspacing loss compensation, thus, according to various embodiments thecomplexity of the servo system may be reduced.

In the following, an effect of a fly height change will be described.

FIG. 12 shows a screen shot 1200 of time domain fly height variation fordifferent thermal TIT (Thermal Fly-height Control) heater power on thetop left chart 1202, and the measured average fly height for differentheater power on the top right chart 1204.

Fly height may be controlled by heater power. Changing the heater powerfrom 70 mw to 80 mw may result in about 1.5 nm fly height change. Flyheight change may cause noticeable signal amplitude change, but may besmaller than the amplitude drift.

FIG. 13A shows a diagram 1300 illustrating a relationship between asector number and a peak to peak amplitude of an averaged cycle for afrequency of 50 MHz and various heater powers. FIG. 13B shows a diagram1306 illustrating a relationship between a sector number and a peak topeak amplitude of an averaged cycle for a frequency of 80 MHz andvarious heater powers. A trend of the envelop amplitude change(indicated by arrow 1302 in FIG. 13A and arrow 1308 in FIG. 13B)corresponding to FH (fly height) heater power (for which a change isindicated by arrow 1304 in the legend of FIG. 13A, which also may applyto FIG. 13B) may be that for increasing FH heater power (correspondingto the direction of arrow 1304), the envelope amplitude (for example ofthe peak to peak amplitude) may increase (corresponding to direction ofarrows 1302 and 1308).

In the following, an amplitude drifting in a 50 MHz single frequencysignal will be described. Amplitude may have a repeatable drifting syncto revolution, for example due to disk deformation which has similareffect as fly-height variation. Within each sector, cross-correlationbetween a raw signal and a sinusoid of a same frequency may also drift.

FIG. 14A shows a diagram 1400 illustrating a relationship between asector number and a peak to peak amplitude of an averaged servo cycle.

FIG. 14B shows a diagram 1402 illustrating a cross-correlation of aservo raw signal with a 50 MHz sinusoid.

In the following, use of AGC to normalize amplitude variation will bedescribed, for example using a dual layer AGC (OD).

FIG. 15A shows a diagram 1500 illustrating raw data and 100 cycles AGCnormalized data. FIG. 15B shows a diagram 1502 illustrating raw data and200 cycles AGC normalized data. FIG. 15C shows a diagram 1504illustrating raw data and 300 cycles AGC normalized data. FIG. 15D showsa diagram 1506 illustrating raw data and 500 cycles AGC normalized data.As can be seen from the number in the respective legends from FIG. 15Ato FIG. 15D, the standard deviation (referred to as “std” in thelegends) may drop due to more cycles used to do averaging.

In the following, a reduction of the interference from a fly heightvariation will be described. Fly height variation and disk deformationmay cause variation of magnetic spacing, which in turn may causereadback amplitude variation. Amplitude variation may introduce extranoise into PES signal which may be generated based on average readbacksignal amplitude. To reduce such amplitude variation, AGC may be used tonormalize the readback signal before PES demodulation. Experiment datashows that using 200 cycles of AGC can effectively reduce amplitudevariation of Servo burst signal from ˜3% to 1.2% (normalized standarddeviation).

The differential servo burst according to various embodiments may reducethe RRO in the PES demodulation noise. The interference from the buriedservo burst may significantly degrade the BER performance of the datachannel. The differential servo bursts according to various embodimentsmay be used to minimize the interference of the servo signal to the datachannel. The layout of the differential servo bursts may not induce thetransition shift to the data magnetic transition while writing datasignal, such that the frequency of servo bursts may be lower which mayreduce the spacing loss and improve the quality of PES. An AGC signalmay be used to normalize the signal of servo bursts, such that tominimize the effects due to the fly height modulation.

For buried servo, the phase alignment can be done at the AGC part.According to various embodiments, the digital PES demodulation withdifferential servo bursts according to various embodiments may beimplemented. The differential servo bursts according to variousembodiments may reduce the interference from the servo layer to the datachannel. When on track, there may no interference from servo channel todata channel. The layout of the differential servo bursts according tovarious embodiments may not induce the transition shift to the datamagnetic transition while writing data signal, such that the frequencyof servo bursts in servo layer may be lower which can reducing thespacing loss and improve the quality of PBS. According to variousembodiments, the AGC signal may be used to normalize the signal of servobursts, such that to minimize the effects due to the fly heightmodulation. The differential servo bursts according to variousembodiments may reduce the coupled data signal and thus may reduce theRRO of PES noise it induced. Performance analysis shows that thedifferential servo pattern according to various embodiments has 3 dB SNRgain over the dual-frequency servo system and the orthogonal servosystem according to various embodiments.

While the invention has been particularly shown and described withreference to specific embodiments, it should be understood by thoseskilled in the art that various changes in form and detail may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims. The scope of the invention is thusindicated by the appended claims and all changes which come within themeaning and range of equivalency of the claims are therefore intended tobe embraced.

1. A recording medium, comprising: a dedicated servo layer configured toprovide servo information, wherein the dedicated servo layer comprises aplurality of tracks, wherein a first track comprises a first servosignal, the first servo signal including first servo bursts of apre-determined frequency, and wherein a second track adjacent to thefirst track comprises a second servo signal, the second servo signalincluding second servo bursts of the pre-determined frequency.
 2. Therecording medium of claim 1, wherein the first servo bursts areorthogonal to the second servo bursts.
 3. The recording medium of claim2, wherein the first servo bursts comprise a sine signal; and whereinthe second servo bursts comprise a sine signal phase shifted compared tothe first servo bursts.
 4. The recording medium of claim 2, wherein thefirst servo bursts comprise a sine signal; and wherein the second servobursts comprise a cosine signal.
 5. The recording medium of claim 1,wherein the first servo bursts are negated to the second servo bursts.6. The recording medium of claim 5, wherein the first servo burstscomprise a sine signal; and wherein the second servo bursts comprise asine signal phase shifted compared to the first servo bursts.
 7. Therecording medium of claim 5, wherein the first servo bursts comprise asine signal; and wherein the second servo bursts comprise a negated sinesignal.
 8. The recording medium of claim 1, wherein the first servosignal comprises a plurality of alternately arranged preambles and servobursts; and wherein the second servo signal comprises a plurality ofalternately arranged preambles and servo bursts.
 9. The recording mediumof claim 1, wherein the first servo signal and the second servo signalcomprises information for providing positioning information.
 10. A datastorage device, comprising a recording medium comprising: a dedicatedservo layer configured to provide servo information, wherein thededicated servo layer comprises a plurality of tracks, wherein a firsttrack comprises a first servo signal, the first servo signal includingfirst servo bursts of a pre-determined frequency, and wherein a secondtrack adjacent to the first track comprises a second servo signal, thesecond servo signal including second servo bursts of the pre-determinedfrequency.
 11. The data storage device of claim 10, wherein therecording medium further comprises a data layer configured to recorddata therein.
 12. The data storage device of claim 10, furthercomprising: a reader head configured to read a signal from the recordingmedium.
 13. The data storage device of claim 12, further comprising: aposition error signal determination circuit configured to determine aposition error signal based on the read signal.
 14. The data storagedevice of claim 13, wherein the position error signal determinationcircuit is configured to determine the position error signal based on anaverage of the read signal over a plurality of servo cycles.
 15. Thedata storage device of claim 13, wherein the position error signaldetermination circuit is configured to determine the position errorsignal based on multiplying the read signal with a waveform of thepre-determined frequency.
 16. A method for determining a position errorsignal in a recording medium, the recording medium comprising: adedicated servo layer configured to provide servo information, whereinthe dedicated servo layer comprises a plurality of tracks, wherein afirst track comprises a first servo signal, the first servo signalincluding first servo bursts of a pre-determined frequency, and whereina second track adjacent to the first track comprises a second servosignal, the second servo signal including second servo bursts of thepre-determined frequency, the method comprising: reading a signal fromthe recording medium; and determining a position error signal based onthe read signal.
 17. The method of claim 16, wherein the position errorsignal is determined based on an average of the read signal over aplurality of servo cycles.
 18. The method of claim 17, wherein theposition error signal is determined based on multiplying the read signalwith a waveform of the pre-determined frequency.
 19. The method of claim16, wherein the first servo bursts are orthogonal to the second servobursts.
 20. The method of claim 16, wherein the first servo bursts arenegated to the second servo bursts.